In an IP multicast network, a multicast routing table is generated in each of packet exchanger devices such as routers, using a multicast routing protocol such as PIM-SM (Protocol Independent Multicasting-Sparse Mode). Transfer of multicast traffic such as a multicast packet is handled using a multicast routing table.
In multicast packet transfer, delivery of multicast packets to a multicast group is started and stopped according to a join message (request to join the multicast group) and a leave message (request to leave the multicast group), respectively, based on a multicast routing protocol. Multicast traffic such as a multicast packet is transferred from a sender (or a source) to a plurality of receivers belonging to a multicast group through a delivery tree (or a multicast distribution tree) including a set of delivery paths each communicably coupling the sender and one of the plurality of receivers.
Further, in the multicast packet transfer, in general, a unicast routing protocol is also used together with the multicast routing protocol. The router determines a first priority interface based on the unicast routing protocol, and sends a control message according to the multicast routing protocol, such as join and leave messages described above, via the determined first priority interface. Stating and stopping the delivery of multicast traffic are controlled using a path for transmitting control messages.
During multicast traffic being delivered, in some cases, a line failure may occur on a delivery tree that is a set of delivery paths along which the multicast traffic is transferred, or network topology including the delivery tree may change. In this case, each of routers related to the delivery tree relearns a unicast communication path using the unicast routing protocol. Then, each of the routers again determines a first priority interface corresponding to a destination address of a multicast packet, according to the result of relearning the unicast communication path. Here, the process for determining first priority interfaces for all the routers relating to the delivery tree is also called the convergence of a unicast routing protocol regarding the delivery tree.
Thereafter, transmission of a multicast control packet (a join message) is performed as to the first priority interface of each of the routers. Then, a reception request message is transferred, on a hop-by-hop basis, from the most downstream router to the most upstream router so that each of the routers relating to the delivery tree relearns a delivery path, where the most downstream router is a router that is located at the most downstream point along the delivery tree and accommodates a destination (client) of the multicast traffic, which is also called a last hop router, and the most upstream router is a router that is located at the most upstream point along the delivery tree and is accommodated by a sender (a server) of the multicast traffic, which is also called a first hop router. After the relearning of delivery paths is completed, in other words, after the convergence of the multicast routing protocol regarding the delivery tree, the delivery of the multicast traffic is re-started according to a new delivery tree that has been made by relearning the delivery paths.
Japanese Laid-open Patent Publication No. 2008-28714 discusses a related topic.